Hi R.:
I'm going to give you some radical advice here! We have taken a HUGE number of road trips since our kids were tiny babies. San Diego to LA, San Diego to Las Vegas, Las Vegas to Arizona, New York to Philly to Washington, D.C., Virginia to Indiana...on and on...and on! My husband and I believe that the whole purpose of going by car or train is to slow down the trip enough so that you can actually learn by what you're watching out the window and so you can stop and pursue interesting things along the way.
Here's the radical part: We never allowed our children to watch DVDs or read in the car because we didn't want to rob them of the whole experience. Not only that, but they quickly became such seasoned travelers that they have developed the patience of Job and their attention spans are tremendous! They also understand directions, distance and time...some adults don't even get those things.
During the trip, I have two sets of goodies: stuff for the car and stuff for the breaks and/or hotel rooms. For the car, I pack drinks, snacks and, for babies, teethers, rattles, and small toys. We play books on tape, music, we sing along, play verbal quizzes and guessing games and we talk. As they got older, they got handed good quality video and digital cameras and they could record whatever they wanted. For breaks, we have kites (the foldable kind) and frisbees to play with in parks or rest areas and for the hotel, we have board games and books and art supplies. Since yours are babies, I'd probably take play dough or blocks instead of the board games and increase the running around time at the park. (Your baby will probably sleep through most of the drive, like someone else mentioned!)
We have travelled with other families and childless people who have remarked at how much fun they've had with my interactive kids on trips. I think it's so much more fun to travel with kids who are engaged socially but you have to train them from very early on.
Have a wonderful time on your trip!
M.